The present invention relates to a circuit for feeding current to a grounded user which current is load independent and represents a measuring value.
The East German patent 2,400,86 describes a circuit for feeding a load independent current into user, the current representing measuring value and including a feedback control amplifier with inputs representing desired and actual measuring value and further including a resistance passed through by the load independent current from which the desired value, subject to the control is taken and fed to the control amplifier. The central element of the circuit is the control amplifier comparing at one input voltages representing desired and actual value, and a load independent current is forced into a measuring resistor such that variations across the resistor are detectable as variations in the voltage drop.
A book by Tietze and Schenck, Springer-Verlag, 1971, under the title "Halbleiter-Schaltungstechnik", page 340 shows a similar circuit. The circuit includes a feedback control amplifier with a differential stage and a controllable current source. The control amplifier provides a current I which is on one hand load independent but is determined by a desired value voltage. The current from the source is passed through a user circuit. The circuit as such constitutes a feed back circuit wherein then the desired voltage taken from a current measuring resistor is used as feed back control variable. Users on the other hand which are grounded at one end have the problem that the desired voltage is superimposed upon a push or push or in phase signal that may vary over a wide range. In another book which is in effect updated issue of the aforementioned book but now referred to as Tietze-Schenck 1980 shows in pages 667-670 a discussion of the problem resulting from this push push signal and the problems resulting therefrom. The remedy proposed in that publication, is set forth in Section 2.5.1.2 and includes a separating amplifier which separates signal and ground connection between the transmitter and receiver and to provide a transmitter with a floating connection to ground.